A radio-frequency signal to be transmitted is normally amplified in the front end of a mobile radio before it is emitted via an antenna. The transmission is normally at a variable radio-frequency output power. This is matched, inter alia, to external characteristics such as the distance to a base station at that time.
In modern third-generation mobile radios which operate on the basis of the UMTS (Universal Mobile Telecommunications Standard) mobile radio standard, the radio-frequency output power has to be variable over a very wide range. A so-called VGA, Variable Gain Amplifier, is typically provided for this purpose in the transmission path of a corresponding communication appliance. An external control voltage is normally used to control the gain of this amplifier, with a signal to be transmitted being amplified as a function of this control voltage.
VGAs such as these are normally manufactured as integrated circuits. The use of mass-production methods for production of chips unavoidably means that manufacturing tolerances can occur. Manufacturing tolerances such as these can result in a shift in the characteristic of the VGA, specifically in its output power as a function of the control voltage. In consequence, there is no longer any guarantee of an unambiguous association between the external control voltage and the output power.
The described problem could be solved by calibration controlled by the baseband chip, which produces the external control voltage by means of an analog/digital converter, a so-called ADC. In this case, when the system is started, that is to say when it is switched on or started up, the control voltage is set successively from the minimum to the maximum voltage, and the associated radio-frequency output power is detected in each case. The association obtained in this way between the output power and the respective control voltage required for this purpose is stored in a table in the baseband chip. However, this method reaches its limits when the shift in the VGA characteristic becomes so great that the digitization range of the ADC being used is departed from. If the manufacturing tolerances are too great, it may then either no longer be possible to cover the entire power range, or, alternatively, an ADC with greater resolution must be used, that is to say an ADC with a greater number of bits, which can cover a wider voltage range. However, this is undesirably associated with additional complexity. An additional disadvantage is that calibrations during the manufacture of integrated circuits with a number of measurement points are associated with high production costs, and it is thus desirable to avoid such calibrations during manufacture, or to keep them as minor as possible.
An object of the present invention is to specify an amplifier arrangement and a transmitting arrangement with the amplifier arrangement, in which the amplifier can be adjusted over a wide range, and which arrangement is robust with respect to those manufacturing tolerances which affect the characteristic of the amplifier and for which the calibration complexity during manufacture is as low as possible.